Vehicle steering column control system

ABSTRACT

A control circuit is provided for transferring power and communication signals through a wireless coupling device in a vehicle steering column. The device can comprise a rotary transformer having a primary and secondary magnetic structure respectfully associated with the wheel side and column side of the steering column. A squib power circuit transforms a high energy power signal suitable for firing an air bag from the column side to the wheel side. A low energy power circuit generates a signal from the column side to the wheel side suitable for operating isolated wheel side electronics, such as cruise control and the like. A tone encoder and decoder circuit communicates low level control signals suitable for signaling wheel side commands from the wheel side to the column side. The low energy power circuit and the tone encoder circuit operate simultaneously and continuously without signal debilitating interaction. An alternative embodiment avoids the low energy power circuit and the encoder and decoder circuit and comprises a pulse circuit to impress a test signal on the wheel side to generate a ring signal representative of air bag firing operability and whether a horn switch is depressed.

This application is a divisional application of U.S. Ser. No.08/790,000, filed Jan. 28, 1997, which is a continuation-in-partapplication of U.S. Ser. No. 08/233,685, filed Apr. 26, 1994, now U.S.Pat. No. 5,636,863.

BACKGROUND OF THE INVENTION

This invention pertains to the art of vehicle control systems and moreparticularly to a control system capable of generating an air bagdeployment signal upon a vehicle collision for an air bag stored in asteering wheel/column and for also transferring a biasing power signalto control circuitry in the wheel for communicating selected drivercontrol signals from the wheel through the column to sensor processingcontrol circuitry. The invention is especially applicable to a controlsystem for transferring signals across a rotating interface. The systemis intended to be capable of transferring both an air bag deploymentsignal and driving control (cruise, climate control, etc.) signals fromthe steering wheel to the steering column. However, it will beappreciated to those skilled in the art that the invention could bereadily adapted for use in other environments as, for example, where aplurality of signals of varying frequency and amplitude need to becommunicated through a physically moving part, and in particular a partthat is continually rotating.

When vehicle air bags were initially introduced on the market, it wasnecessary to remove driver control functions from the steering wheel andinstall them on stalks that emanated from the steering column.Typically, the only two items which remained that were wheel-mountedwere the air bag and the horn. The signal for the horn was transferredfrom the wheel to the column through slip rings. The slip ringsconsisted of a ball contact located on the steering wheel and a circularconductor which was part of a "clock spring". The clock spring was amolded plastic part which housed a two conductor ribbon cable thatconnected the air bag to its control module. Such a clock spring iscapable of maintaining electrical connections during rotation of thewheel.

As the demand for placement of driver controls back onto the steeringwheel has become greater, the clock spring was changed to comprise ahousing for a multi-conductor ribbon cable and/or slip ring. Thisarrangement allowed both the air bag and driver control switches tooperate independently on the steering wheel. Some clock springs have asmany as six conductor ribbon cables and no slip rings.

The numerous design concepts comprising adaptations of slip rings andclock springs have been fraught with problems and are of limitedeconomic and practical value. Slip ring arrangements have alwayssuffered from reliability and performance problems due to the inherentnature of the slip ring structure itself. The electrical integrity ofthe contacting methods will necessarily depreciate over time from dirtand/or wear and varying ambient conditions. In addition, the assemblyrequirements for multi-conductor ribbon cables in a steering column havebeen notoriously undesirable for the vehicle manufacturers, not only forthe relatively high expense of the cable and contact componentsthemselves, but also from the labor costs involved in the assemblyoperation.

A particular problem with prior known systems which have placed certaindriver controls back onto the steering wheel concerns the increasedcomplexity in wheel side electronics. Problems with assembly,maintenance and reliability will always arise as the complexity ofcircuity increases in an automobile component, such as a steering wheel.

The present invention contemplates a new and improved method andapparatus which overcomes the above-referred to problems to provide anew vehicle steering column control system which is relatively simple indesign, economical to manufacture and assemble and provides highreliability and performance in deploying both an air bag ignition signaland communication of biasing power and driver control signals both toand from the steering wheel and the steering column.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided electroniccontrol circuitry which accomplishes three major functions:

1. Provide a high power signal to a steering wheel rotary transformersufficient to fire an air bag squib.

2. Provide driver communications (such as cruise control commandsignals) from the steering wheel back through the rotary transformer tothe steering column, for various control purposes.

3. Supply continuous low-level operating power for isolated wheel sideelectronics that comprise the driver controlled communication devices,from the column side, through the transformer, to the wheel.

The system employs a rotary transformer that provides uniform couplingacross a rotating interface regardless of the rotational angle betweenthe primary and secondary magnetic structures. The transformer is of asufficiently small size that it can be practically integrated into asteering wheel/column structure, yet is able to transfer the severalamperes of current required to fire an air bag squib element.

The subject invention more particularly comprises a control circuit fortransferring power and communications to a vehicle steering columncomprising a rotary transformer having a primary and secondary magneticstructure, wherein the primary magnetic structure is associated with acolumn side of the steering column and the secondary side of thesteering column is associated with the wheel side of the steeringcolumn. A first circuit portion transfers a high energy power signalsuitable for firing an air bag from the column side to the wheel side. Asecond circuit portion transfers a low energy power signal from thecolumn side to the wheel side suitable for operating isolated wheel sideelectronics. A third circuit portion communicates low-level controlsignals suitable for signaling wheel side commands from the wheel sideto the column side, wherein the second and third circuits cansimultaneously operate continuously without signal debilitatinginteraction. The high energy power signal transferred by the firstcircuit is of such energy that it will, of course, overwhelm the othersignals, but in the condition of a collision, wherein the air bag isenabled, wheel side control signals will be irrelevant.

In accordance with another aspect of the present invention, the firstcircuit portion includes a fire switch associated with the column sidewhich is enabled during the vehicle collision and a squib switchassociated with the wheel side for connecting the high power signal to asquib element in a firing condition for the air bag. The squib switchincludes a means for isolating the squib element from the low energypower signal and the low level control signals in normal operatingconditions to preclude shunting of other functions thereof by the squibelement.

In accordance with a further aspect of the present invention, the highenergy power portion signal is tuned to a frequency detectably spacedfrom a frequency of the low energy power signal and the high energypower signal has a voltage level detectably spaced from a voltage levelof the low level control signals. A filter segregates the low energypower signal and the low level control signals from the squib element.The filter comprises a frequency band filter for attenuating the lowenergy power signal and a level detection circuit for excluding the lowlevel control signals.

In accordance with a more limited aspect of the present invention, thefirst circuit portion includes a squib power circuit associated with thepower switch for generating the high power signal. The squib powercircuit includes a test circuit for testing if the squib element ispresent and capable of receiving the high power signal at vehicle startup. Power up is detected from the control circuit and a pulse issupplied to the squib element of the high power signal at a selectivelyreduced power level selected to be below the ignition point of thesquib, for exercising and demonstrating the full function of thecircuit.

An alternative embodiment of the present invention contemplates a methodand apparatus which can produce a high energy signal communicatedthrough a contactless device like a rotary transformer that is suitableto deploy the airbag, and which further includes a test circuit forapplying a test pulse across the transformer device and sensing a secondsignal in response to the pulse, which signal is selectivelyrepresentative of the operability of the airbag firing signal circuit.Further, a horn switch circuit is operable in combination with the testpulse signal, so that activation of the horn switch alters theresponsive signal of the pulse in a manner which is also identifiable bya column side electronic circuit in order to actually activate the horn.This alternative test circuit and signal consumes a relatively low levelof power and so can be continually operable for monitoring airbagoperability and horn activation, even when running on mere batterypower.

One benefit obtained by the present invention is an air bag controlsystem which allows a transfer of control and power signals across arotating interface with structural simplicity and high reliability.Previous structural requirements of multi-ribbon cable and slip ringconductors are avoided.

Another benefit obtained from the present invention is an air bag firingcontrol system which is economically more efficient in assembly andstructural cost.

A further benefit of the present invention is a steering column controlcircuit which employs signal control circuitry (DTMF) with provenperformance achievements for enhanced reliability and performance, incombination with a signal transferring rotary transformer which provideda non-contacting signal transferring method.

A benefit of an alternative embodiment of the invention incorporatingminimal wheel side electronics, e.g., a horn, allows for monitoring ofairbag operability and horn activation by mere detection of response oftransformer secondary side "ringing" in response to imposition of a testpulse from the primary side. Such a system allows for continualmonitoring of the airbag and the horn, even by mere battery power, aswhen the vehicle is shut off. Accordingly, the subject inventionprovides substantial benefit and safety by continually monitoring theairbag firing system.

Other benefits and advantages for the subject new control system willbecome apparent to those skilled in the art upon a reading andunderstanding of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take physical form in certain elements andarrangements of elements the preferred and alternative embodiments ofwhich will be described in detail in this specification and illustratedin the accompanying drawings which form a part hereof and wherein:

FIG. 1 is a block diagram of a steering wheel control system formed inaccordance with the present invention;

FIGS. 2A-2D comprise a detailed schematic of a circuit of the presentinvention;

FIG. 3 comprises a schematic representation of a rotary transformerincluding concentric coils that can be utilized as part of the presentinvention;

FIG. 4 comprises a simplified schematic for identifying the operatingprinciples of an alternative embodiment;

FIG. 5 comprises a diagrammatic illustration of a ring signal generatedduring normal operating conditions for the system of FIG. 4;

FIGS. 6 is a view similar to FIG. 5 wherein the squib element nowoperates as an open circuit;

FIG. 7 is a view similar to FIG. 5 wherein the squib element operates asan electrical short;

FIG. 8 comprises a diagrammatic illustration of a ring signal when thehorn switch is depressed;

FIG. 9 comprises a block diagram of an excitation signal extractioncircuit formed in accordance with the principles of FIG. 4;

FIG. 10 is an output signal for monitoring the ring signal by a countingprocessing technique;

FIG. 11 is a diagrammatic output of a processing technique wherein thering signal is integrated;

FIG. 12 is a diagrammatic signal representative of an output of anotherprocessing technique wherein the ring signal is output to a sample andhold circuit and then integrated;

FIG. 13A-C is a detailed schematic of an airbag firing circuit formed inaccordance with the alternative embodiment of the present invention; and

FIG. 14 is a detailed schematic of another type of processing circuitfor a portion of the circuit of FIG. 13 comprising a sample and holdintegrator system.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein the showings are for purposes ofillustrating the preferred and alternative embodiments of the inventiononly, and not for purposes of limiting same, the FIGURES show a controlcircuit particularly intended for use in a steering column of a motorvehicle (not shown). The control circuit transfers power andcommunication signals through a steering column comprised of aselectively rotatable wheel and a fixed steering column. The circuit isbasically comprised of a common signal coupling device forcontemporaneous parallel transmission of the signals through the vehiclesteering column, preferably shown as a rotary transformer 10 having aprimary and secondary magnetic structure, wherein the primary magneticstructure is associated with the column side (FIGS. 2A and 2C) of thesteering column and the secondary magnetic structure is associated withthe wheel side of the steering column (FIGS. 2B and 2D). A first circuitportion 20 (FIG. 2C, shown in dashed line), transfers a high energypower signal suitable for firing an air bag (not shown) from the columnside to the wheel side. A second circuit portion 30 (FIG. 2A) transfersa low energy power signal from the column side to the wheel sidesuitable for operating isolated wheel side electronics and decodesdriver generated control signals. A third circuit portion 40 (FIG. 2D)communicates low level control signals suitable for signaling wheel sidecommands (cruise, climate control and the like) from the wheel side tothe column side. It is a feature of the invention that the second andthird circuit portions can simultaneously operate through thetransformer 10 regardless of rotational position of the wheel relativeto the column and continuously without signal debilitating interaction.The output circuit portion 20 generates a relatively high power signalto ignite a squib 50 for deploying an air bag to protect the driver ofthe vehicle in a collision. Other elements represented in the blockdiagram of FIG. 1 comprise the high power solid state switch 60 whichfires the squib 50 and a test circuit 70 which tests the operability ofthe squib firing circuit without actually firing the squib, such as atvehicle start up.

During normal operation of the circuit (non-air bag firing operation),only communication functions are required through the steering columnfrom control switches on the wheel side back through the transformer 10to the column side. Although to accomplish these communicationfunctions, power must be transferred for the wheel side electronicscontinuously from the column side. A thirty Khz generator 32 generates asine wave shaped power signal. A sine wave shape is used to minimize EMIinterference to the vehicle system. A high pass filter 34 amplifies thesignal to a level of several volts into the primary of the rotarytransformer 10. With particular reference to FIG. 2A, the detailedschematic of this portion of the circuit is illustrated wherein afunction generator integrated circuit 36 (such as an ICL 8038) generatesthe sine wave shaped signal, which is then amplified by a poweramplifier integrated circuit 38 (LM 380) to the level of several voltsinto the transformer primary.

This power signal is coupled to the secondary winding of the transformer10 as a low energy power signal for tone encoder circuit 40. Inparticular, the signal is fed through a current limiting resistor 45(FIG. 2D) and a rectifier diode (1N 4934) 46 into a one micro faradcapacitor 47. The DC voltage formed across the capacitor is the lowpower DC voltage source (VCC) (FIG. 1) for the wheel side electronics.The high, 30 khz frequency assures that the primary winding reactancewill be high enough to keep primary current to a relatively low level,since only low power transfer is necessary for normal signalcommunication operation. Also, the 30 khz signal can then be filtered orseparated more easily from the low level communication signals whichoccur between 697 and 1645 hz and the high power air bag firing signalwhich is set at 2 khz. Although the preferred embodiment discloses thelower power signal to be 30 khz, of course, other frequencies can beused so long as they are capable of being separated from the otherfrequencies with the appropriate filters.

Communication signals for the wheel side switch commands areaccomplished with a standard telephone keypad encoder integrated circuit42. This dualtone/multi frequency encoder (D.T.M.F.) merely convertsswitch closure commands into tone signals ranging from 697 hz to 1,645hz which are then coupled back through the rotary transformer secondaryback to the column side primary winding where tones are extracted,amplified and decoded for control purposes. These are importantoperational features of the subject invention since the tone signals areof a very low level of only a few millivolts in the presence of severalvolts for the 30 khz low energy supply signal. This operationaladvantage is accomplished as follows. First, the signal at thetransformer primary is fed to a "Twin T" notch filter 43 tuned to 30 khzwhich provides significant attenuation of the 30 khz signal, and allowsthe communications tone signals to pass virtually unattenuated. Thefilter output is then fed to an additional RC filter. The 30 khz signalhas now been reduced below the level of the tone signals. Although thetone signals are still at a very low level, at this point, they areamplified by an amplifier 44 (LM 386) for voltage amplification to ausable level. The output of the amplifier 44 is then fed to a two-stageRC filter 49 for additional filtering of any 30 khz components which mayhave coupled through. The now "clean" amplified tone signals are fed toa DTMF decoder 48 (75T204) and are converted to logic signals. The logicsignals are fed to a multiplexer interface 99 and then to theappropriate digital signal processing circuitry through the vehicle MXbus. Both the DTMF encoder 42 and decoder 48 are crystal controlled forextreme stability under all conditions and require no tuning oradjustments. Such components enjoy wide proliferation for telephonebased applications, and are accordingly inexpensive and reliable.

Although the system has been described as using DTMF encoders anddecoders, other communication schemes are within the scope of theinvention and have been considered and evaluated with success, so longas proper filtering is accomplished for isolation of the power supplysignal frequencies. Such other methods involve modulating the tonefrequencies onto the power supply signal using it as a "carrierfrequency" (rather than linearly mixing them), or using frequency shiftkeying (FSK) techniques. Such other alternative techniques havesuccessfully demonstrated bidirectional communication.

Air Bag Firing Condition

An air bag firing condition is evoked if a collision occurs. Then aswitched command signal, i.e., fire switch 80, applies power to a highpower inverter circuit 22. A square wave signal of approximately 2 khzis imposed onto the transformer primary winding through a high current"H-drive" circuit 22. At this frequency the transformer 10 is capable ofcoupling large amounts of power. If a low impedance load, such as theair bag squib 50, is coupled across the transformer secondary, thereflected low impedance on the primary will produce very high primary(and secondary) currents to flow, but only if an adequate power drivercircuit is used.

The H-drive alternately switches the primary with forward and reversepolarity square wave signals equal to the system DC power applied to it.With a conventional 12 volt vehicle system, this is the equivalent ofapplying a plus and minus 12 volt square wave (24 volts peak-to-peak)onto the transformer 10, yet only using a single ended 12 volt supply.Accordingly, primary currents will exceed 20 amperes and secondaryvoltages and resultant currents into the squib are sufficient to fire anair bag.

In the embodiment shown in FIG. 2C, the square wave signal is initiallygenerated by a 2 khz generator circuit 24 (TL 494 integrated circuit),especially designed for inverter and PWM power supply applications. Twoalternating square wave signal outputs are generated to be used to drivethe H-drive circuit 22. The H-drive output is comprised of four highcurrent FET power transistors 91, 92, 93, 94, two P-channel types andtwo N-channel types. For operation, for instance, transistors 91 and 94are to be turned on to conduct current through the transformer primaryin one direction. In effect, one side of the winding is connected toplus 12 volts and the other side to ground. On the next half-cycle, thisis reversed by turning off transistors 91, 94 and instead turning ontransistors 92, 93, and in effect reversing the polarity on thetransformer. The generator circuit 24 is operated in a mode that allowsa brief "OFF" period between polarity reversals to assure that two FETson one side of the bridge are not simultaneously turned on, which couldcause a short across the main supply to ground (i.e., transistors 91 and93 are never on simultaneously, or transistors 92, 94 either). Logiccircuity 26 between the generator 24 and the H-drive 22 is used toswitch the H-drive 22 from a not operational state (normal systemoperation) to a fire condition. In other words, during normal operationof the steering control system, the H-drive 22 should not be operatingin the high power inverter mode. Rather, a transistor 94 is the only FETbiased on, and this is done to ground at one end of the transformerprimary, which is necessary for normal operation. The other three FETS91, 92, 93 are held in an OFF state by the logic circuitry, leaving theinput side of the transformer primary effectively open for the normalcommunications low power supply signals to operate unhindered.

Another feature shown in the detailed schematic of the subject controlsystem is that the reverse, source to drain substrate diode built intothe FET 92 tends to clamp the input signals when they swing negativewith respect to ground. This is circumvented in the circuitry shown byusing a transorb D22 (high power zener diode 1N6283) in the source toground connection of FET 92. The zener diode prevents reverse currentflow from ground up to the source (and substrate diode) of FET 92 duringthe reverse polarity phase of the input signal (the 30 khz signal) tothe transformer primary. When the H-drive 22 is in its high powerinverter mode, the diode is essentially "transparent" to the circuitexcept for its forward bias voltage drop of approximately 0.6 volts.Because the current flow is high in the fire mode, the diode must becapable of handling this current and this is why a large zener(transorb) is used. In order to decrease the diode voltage drop in theforward direction, a "Schottky" diode D21 is shown in parallel with thezener. This can help reduce the forward voltage drop further, down to0.2 to 0.3 volts. In the embodiment illustrated, five smaller Schottkydiodes (1N5817) were used, but a single larger device would beappropriate.

A further feature of the subject inventive circuit is the use of a"one-shot" or monostable circuit 28, comprised of an integrated circuit(4538). When a fire sequence occurs, the logic steers the H-drive 22into its high power inverter mode, the one-shot circuit 28 is triggeredand causes this to occur for only a short period (200 ms), even thoughthe fire input command from the switch 80 may stay on for an extendedtime. This is because only a brief period is required to fire the squib50. This one-shot feature has the advantage of protecting thetransformer from being driven for an extended period of time, whichcould overheat its windings.

Squib Firing Switch

On the secondary side of the transformer, under normal conditions thesquib element must be isolated from it or its low resistance will shuntout the low power supply and tone communications signals. Yet, when afiring mode is to occur, the squib must be switched across the secondaryby use of a low resistance, high current switch. This is accomplished bythe circuit shown under "2 khz Squib Switch" 62 (FIG. 2B). Here, a solidstate, high current AC 62 switch is used to isolate the squib from thetransformer secondary under normal operating conditions. Two,back-to-back, N-channel power FETs, 96 and 97, are configured to act asan AC switch. In one direction of the AC line, one of the FETs will havepositive polarity across its drain to source connections, and the otherFET will be reversed. The forward biased one will carry current in thenormal direction from drain to source (when its gate to source is biasedon), and the reverse FET will carry this current through its integralreverse source to drain substrate diode, even though it is not operatingas a normal transistor in this mode. When the line polarity is on theother 1/2 cycle of the AC line, the roles of each FET will be reversed.Whenever a DC gate voltage is applied to the two FETs from the commonsource connections to the common gate connections, the AC switch will beon, and when the gate signal is removed, they will both be off, blockingcurrent conduction in both directions. With the present circuit, the ACswitch is connected to the high side of the squib load, which means thatwhen the FETs are in an "on" condition they will "float" above theground level along with the incoming power signal from the transformersecondary. Thus, to assure that the FET 96 gate is biased to at least 5volts above the common source connections (to assure that FET 96 is heldon), the bias signal with respect to ground must exceed the highestpositive polarity voltage level of the AC signal by at least 5 volts. Toderive this, a voltage booster circuit is used. A small transformer X2,is connected across the rotary transformer secondary to provide anadditional power source through diode D7 and capacitor C35, currentlimiting resistor R61 and zener diode D8. Alternatively an additionalsecondary winding (of smaller gauge wire) could be incorporated into therotary transformer to provide the step-up. The negative end of thisisolated supply is then connected to the positive side of the normalwheel side power supply (Vcc), to provide the boosted voltage which isthen supplied to the emitter of PNP transistor Q13. Whenever thistransistor is turned on, it will tie the boosted voltage to the FETtransistor gates through limiting resistor R51, turning on the FETs.Zener diode D10 prevents the gate to source voltage from ever exceeding16 vdc, however, protecting the gate circuits from excessive voltage,particularly when the power AC signal polarity is in the reversedirection. The base of Q13 is, in turn, driven by the collector circuitof NPN transistor Q12 through resistor R55. Q12 is toggled on and off bythe logic output of the level detector/comparator stage of U14b as partof the level detection 66. This is accomplished as follows: First, the 2Khz firing signal on the secondary of the rotary transformer must bedistinguished from the 30 Khz power supply signal, both of which haveappreciable voltage level swings, but differ significantly in frequency.Secondly, the 2 Khz signal must be distinguished from the communicationstone signals which are very close in frequency, but much lower inamplitude. The prototype circuitry accomplishes this by using a sharp,frequency selective "low-pass" 64 filter to attenuate the 30 Khz signalto a very low level, essentially rejecting it, yet allows the lower, 2Khz firing signal frequency to pass through unattenuated. The low passfilter 64, shown in FIG. 1 also, is a 2-pole, active filter comprised ofoperational amplifiers U14a and associated components. Although the 30Khz signal has now been practically eliminated, the communications DTMFtone signals have not, and under normal operating conditions, these willpass through the filter. Because these tone signals are very low inamplitude they can be excluded with a level detection circuit 66 afterthe filter 64. This is accomplished by rectifying the output of thefilter with diode D9, and smoothing it with capacitor C39, then feedingthis to a voltage comparator circuit comprised of U14b, and referencedivider R47, VR3 and R48. The divider sets up a DC voltage level thatthe filter output AC peaks must exceed before U14b will provide a logicsignal for firing. Only the 2 Khz squib fire signal has both the lowfrequency and high amplitude level to pass through the filter, then berectified to a DC level (by the 1N4148 diode D9 and the 0.1 mfd C39) toproduce a DC voltage adequate to toggle the comparator which is thenused to drive the gate circuit of the output AC switch, thus connectingthe squib to the transformer. Note that the filter/threshold detectorcircuit normally operates at a very low power level, with U14 being a"micropower" type IC, assuring minimal loading on the low power 30 Khzpower supply link. Whenever a high power 2 khz squib firing burst comesacross the transformer, the available power supply is now dramaticallystiffened and the switch control circuit can be allowed to draw moreoperating current to turn on, this occurring when Q12 and Q13 are turnedon to drive the FET gates (i.e., the squib firing switch circuitnormally idles at very low current levels until a firing sequenceoccurs, at which time it draws more current to bias on the FETs,although this is nothing compared to the current then being switched tothe squib).

Built-in Test

With most air bag systems, a test function is invoked at vehicle startup. With such a test function, the plan is to detect power up andbriefly pulse the squib firing sequence by enabling the 2 khz powerinverter 22, but at a power level below the ignition point of the squib,while checking that the squib 50 is receiving the inverter signal. Thesubject invention accomplishes this task by reducing the duty cycle ofthe inverter signal. Segments of the square wave having nearly 50% dutycycle on the positive polarity poles and 50% on the negative poles, thepulses are only on for approximately 5-10% of the normal ON time. Thisis accomplished with the wheel side built in test circuit 70 and thecolumn side built in test circuit 90. The column side built in testcircuit includes a system power up detect circuit 92, a PWM duty cyclecontrol 94 and a relay driver logic circuit 96 which operates the relayfor controlling the contacts across the air bag switch 80. Withparticular reference to FIG. 2C, the column side built in test circuitis accomplished with the TL494 generator chip, because it is designedfor pulse width modulation control. For instance, by varying DC voltagelevel on the TL494 generator chip, the output duty cycle can be variedfrom virtually 0 to 100%.

Thus, for the power up test, the total squib firing circuitry comprisingtransformer 10, squib firing circuit 20 and squib switch 60 can betested without applying enough heating duty cycle to the squib 50 tofire it. This reduced duty cycle pulse train can be detected across thebuilt in test circuit 70 comprising sample and hold type circuit 72,then fed as a logic signal from logic circuitry 74 to the tone encodercircuit 40, and then transmitted as a tone signal back to the columnside decoder 48 as an "OK" signal for the bag test function immediatelyafter the power pulse train terminates. Such a test scheme provides muchmore confidence in the total air bag firing system than a simple testthat merely looks at the resistance of the squib 50 to see if it ispresent.

As noted above, the subject system was faced with the particular problemof integrating all circuit functions for successful continuous operationtogether without interaction or damage. The synergistic operation of thesubject inventive circuitry provides an air bag control and firingcircuit with substantial advantages over prior art systems.

Some further comments which will be appreciated by those of ordinaryskill in the art with regard to the detailed schematic shown in FIGS.2A-2D comprise:

The 10 ohm current limiting resistor R57 and the 1N4739 zener diode D14voltage clamp in the wheel side power supply circuit are mainly forprotection when the 2 khz power inverter signal is present.

The capacitor between the output of the 20 khz driver circuit and thetransformer primary cannot be too large, or its reactance, inconjunction with the low output impedance of the LM380 will shunt(attenuate) the tone frequencies coming back from the wheel side. Also,the 2 khz inverter signal would be shunted through this branch if thecap were large. Ideally, a more effective "high pass" filter should beused in this spot.

Extracting the low level tone signals in the presence of the high level30 khz signal takes quite a bit of filtering, as described previously.

Getting the H-drive circuit to work in two or three modes, and notaffect normal operation was a challenge without using a relay contactorfor the switching, which would have presented timing problems as well ascost and reliability.

The transformer had to primarily be designed to efficiently transfersquib firing power from the column to the steering wheel. It wasoptimized to do this at about 2 khz. The other functions and circuit hadto be tailored to work with this transformer. A primary to secondaryratio and wire sizes which were ideal for high power transfer were notideal for the low power transfer and communications functions.

Although the continuous running, low energy power supply signal wastransferred with a sine wave shape to minimize EMI generation, the highpower firing signal was generated as a square wave signal forefficiency. Any brief EMI noise burst during an air bag firing scenariowould not be of concern. The active filter use in the squib firingswitch could also be accomplished in other ways, but whatever filter isused, it must be capable of processing several simultaneous signals atwidely different amplitude levels. Some digital filters work well withsingle component signals, but cannot handle combinations. Analogfiltering, as used, works well. This also applies to the filtering usedon the input of the DTMF tone decoder circuitry.

The H-drive circuit was required as maximum power was to be coupled to asingle winding. If a center-tapped, dual 12 volt winding could have beenused, the driver could have been a simple, two transistor "push-pull"output. Winding space considerations in the prototype transformer wouldnot allow this, so forward and reverse drive of a single winding becamenecessary, hence the H-drive configuration.

With references to FIG. 4, an alternative embodiment of the subjectinvention is disclosed particularly directed to minimizing wheel sideelectronics so that mostly passive components are utilized on the wheelside. In other words, the second and third circuits for communicating alow energy power signal or low level control signals from wheel side tocolumn side through the transformer 10 are not necessary. Thisalternative embodiment is directed to merely monitoring on the primaryside of the transformer a reflected signal from the secondary side forpurposes of identifying the operability of the airbag squib circuit onthe secondary side, and whether a horn switch has been actuated on thesecondary side. Accordingly, a contactless electrical link between thesteering column and the steering wheel provides for signal diagnosis ofthe operability of the airbag firing circuit back to the column, as wellas deployment thereof, and further includes a circuit for sounding ofthe horn in response to closure of a horn switch on the wheel sidecircuitry. All of these objectives are accomplished with only passiveelectrical components on the secondary side of the rotary transformer.Thus, the subject alternative embodiment is directed to theimplementation of the rotary transformer as a contactless link betweenthe wheel and column in a system providing essential system requirements(the horn and airbag testing), while incorporating only a minimum oftask requirements on the wheel side.

The subject alternative embodiment involves the method of brieflypulsing the transformer primary winding and observing a subsequent"ringing" signature to derive information about conditions in thesecondary winding on the isolated steering wheel.

In certain preexisting systems, resistance of the loop through the clockspring and squib was monitored with a low level DC current to ensurethat the airbag could be fired when necessary. Since the rotarytransformer 10 of the subject invention does not provide this directelectrical connection, the sensing process for monitoring secondary sideconditions becomes more difficult.

The subject invention desirably concentrates most of the necessarycircuitry on the column side for thereby minimizing wheel side circuitcomplexity. The invention further includes the transformer itself as apart of the monitoring process. The monitoring is accomplished at a lowpower level which minimizes the chances of unintended squib 50 ignition,electrical system loading and generation of electromagneticinterference. In the subject disclosed embodiment, in addition todetermining squib operability conditions, the invention provides theability to detect closure of a horn switch, which is also located on thesteering wheel. Horn switch detection is accomplished regardless of thestatus of the squib condition on the secondary of the transformer,whether it is shorted, normal, or open.

Further, with this technique, operating power can be low enough toprovide horn switch detection when the vehicle is not running withoutconcern of battery depletion. Of course, neither the squib monitoring orhorn detect functions will interfere with the essential function ofairbag firing.

The method and apparatus of the invention works in the following manner.Referring to FIG. 4, the transformer primary winding 100 is excited witha current pulse 102 from a finite impedance source 104. A voltage pulseproduced at the secondary 106 then feeds through a capacitor C4 into thesquib resistance element 50 resulting in a current flow 108. Theresulting current flow 108 recirculates as a resonance between thecapacitor C4, the squib 50, and the transformer inductance 106, and diesout over time due to the energy being dissipated in the resistance inthe loop. This combination "rings" each time a pulse occurs. Amechanical analogy might be that of striking a bell with an impulse froma hammer, after which the bell rings at its own resonant frequency ortone. It eventually dies out. If the bell's ringing energy is absorbedby some damping action, such as someone holding it, it will still ringat the same frequency, but the ringing will die out in a shorter time.

With this technique, the shape 108 of the ringing envelope, particularlyhow it is dampened, is a direct function of the resistance (squib) load50 in the secondary winding loop. As the resonating current 108 goesthrough the resistance, power is dissipated so the resonance will dieout. In effect, the squib resistance 50 affects the "Q" of the ringingcircuit, with changes that are quite apparent. By "Q" is meant thequality of the frequency response that is quantitatively defined as theratio of the resonant frequency to the band width. In terms of energy,Q=2π·(maximum energy stored/total energy lost per period). The ringingcurrent 108 on the secondary winding is reflected back to the primarywinding where it can be observed as a similar current 110 or voltagesignal. The source resistance 112 of the driving circuit on the primaryis high enough to minimize its effect on the damping action. FIGS. 5, 6and 7 show changes in the ringing signal voltage at the primary 100 witha secondary normal squib resistance and an open squib (FIG. 6), orshorted squib (FIG. 7). As can be seen, as resistance decreases, theringing signal will be more energetic, i.e., taking a longer time todampen out, as there are less resistive losses to absorb the energy. Inall cases the ringing frequency itself remains approximately the same,or about 25 Khz, but the damping rate changes, and will generally becomegreater with increasing values of load (squib) resistance. The initialring amplitude is the same because it is really the initial drivingpulse imposed onto the primary when transistor Q1 (FIG. 13A) turns on.The subsequent ringing is really where the load differences manifestthemselves. The shape of the damping can be directly correlated with thesquib resistance 50, thus with approximate processing, the squiboperability and condition can be determined. The processing techniqueused would depend upon the degree of accuracy and resolution requiredfor monitoring or testing. A simple method involves counting the numberof ringing cycle peaks that exceed a minimum amplitude, following eachexcitation pulse. Improved resolution is achieved through integration ofthe amplitudes of the successive ringing cycles. Even more sophisticatedsignal processing can derive the exact damping factor by comparing thepercent amplitude decrease of each successive decaying ring. Doing somakes the measurement even less dependent on the exact magnitude of theexcitation pulse, transformer air gap size, or other particular circuitfeatures.

The capacitor C4, if left alone in series with the squib 50, would limitthe drive signal into the squib when the air bag is to be deployed. Thisis easily solved with two parallel rectifier diodes, facing oppositedirections, connected across the ringing capacitor as will be shownschematically below. If the ringing signal remains below the conductionvoltage of the diodes, the signal behaves as if the diodes were notthere. In the event that the squib is to be deployed, the actual highamplitude firing burst through the transformer will far exceed thatnecessary to forward bias the diodes, delivering high current to thesquib with minimal loss. Another successful method employs a small"saturable reactor" inductance in place of the diodes. This reactor isdesigned so that its core is excited only within its linear range by thelow amplitude, relatively high frequency of the ringing signal. In thismode, it presents a high reactance, allowing the parallel capacitor toring with the transformer. If the air bag is to be deployed, however,the high amplitude, lower frequency inverter firing signal quicklysaturates the core on the leading edge of each half cycle, dramaticallylowering the inductance (to that of the coil with an air core, i.e.,essentially only the resistance of the coil winding) thus passing thedesired high current to the squib.

In conjunction with the above described squib condition sensing,detecting horn switch 200 closure is possible by using the switch toconnect another capacitor C10, across the secondary winding andobserving the change in frequency of the ringing signal. FIG. 8 showsthe change in the ringing waveform with a 1 mfd capacitor C10 across thesecondary 106. Comparing this with the normal ringing of FIG. 5, one cansee that the ringing or resonance frequency is lowered, in this casefrom approx 25 Khz down to approx 14 Khz, a significant change which iseasy to detect. What is particularly significant is that it is workablewhether the squib is at normal resistance, or shorted or open, makingthe horn function independent of squib condition. Also, if the horn 200is switched on at a time that the air bag has to be fired, a fairlylikely scenario, the capacitor C10 does not appreciably divert deploycurrent from the squib 50. Any concern that a shortened capacitor mightbe switched in, shunting current from the squib 50, could easily bealleviated through use of a small, low current fuse in series with thecapacitor, which would blow instantly under such conditions. Normal hornoperation ringing currents would be far below the fuse blow point. Aresistance could also be used in series with the capacitor C10 to limitthe loading of a shorted capacitor.

In practice, additional switch functions could be detected merely byswitching in various capacitor values, and detecting the ringingfrequency changes.

It should also be emphasized that the ringing behavior is the samewhether it is invoked rapidly or at a slow rate, just like the bellringing example. With this approach, a typical ringing burst occurswithin a very short interval of, for example, only 500 microseconds,from which all the desired conditions can be observed. The system testupdating rate requirements might only be once every several seconds,although a practical rate for horn button detection would be more like 5to 10 times per second. Whatever the requirements, this technique canoperate at rates limited only by the ring out test duration, or close to2000 times per second. At slow rates the power consumption can be verylow, permitting functions such as horn detection to be operational evenwith the vehicle shut down.

With reference to FIG. 9, a block diagram is shown which isrepresentative of a column side circuit system for accomplishing primaryexcitation and signal extraction in accordance with the ringing methodof FIG. 4. Although there are a plurality of processor techniques whichcan be implemented in the block diagram, some of which will be discussedbelow, all can be explained with reference to FIG. 9.

The ignition block 802 provides the appropriate signal to turn on thedeploy oscillator, indication of ignition on and normal operation. Thedeploy oscillator 804 is the main (squib) square wave oscillator fordeployment modulation. The modulation logic block 806 modulates theprimary 100 of the transformer 10 at a normal frequency of 5 hz and willswitch to a 4 khz signal when a deployment comment is received from theairbag electronic control unit 808. The pulse generator 810 provides a500 msec on time and a 200 mmsec off time pulse used to "ring" theprimary 100. This ring signal is processed in accordance with thesubject invention to monitor squib condition and horn switch activity.The FET 812 is the active element in the circuit responsible for primarymodulation. The BIT and horn logic block 814 will operate in accordancewith the various methods described in detail below. The "BIT" (Built-inTest) of status block 816 is responsible for displaying squib status.The horn driver block 820 will sound the horn when the horn switch isdepressed on the wheel side and is detected through the logic block 814.It is a feature of the subject invention that the secondary electronics822 are all passive components for circuit minimalization of wheel sideelectronics.

Method 1: Comparator Threshold Processing

There are two ways to accomplish this processing technique. One way isto count pulses and determine secondary activity based on specificnumber of counts. Anther method is to integrate the pulses and obtain afinal voltage based on the total number of pulses reflected.

Counts

If you take the negative peaks (positive peaks could also be used), asshown in FIG. 5, and input those into a window comparator, you would geta square wave as shown in FIG. 10.

Each peak of the square wave would be counted as 1, and the total"count" compared to the preset values for squib condition and hornswitch activity. For example, FIG. 10 is the "count" for a normal squibwith horn switch open, so a count of 6 would equal this condition. Thereare different counts for each condition which is based on the detectedring signals.

In FIG. 6, the count would be 1, indicating an open squib, horn switchopen. In FIG. 7, the count would be 10, indicating a shorted squib, hornswitch open.

Pulse Integration

This technique starts with the same circuit as the count method. Thatis, the ring signal is an input to a comparator which produces the samesquare wave as mentioned above. However, this square wave is now putinto an RC network which acts as integrator. The final voltage on thecapacitor is a function of the number of counts or peaks of thecomparator output. FIG. 11 is an example of an integrated signal whichis a normal squib with horn switch open. Because the pulse widths becomenarrower at the decaying end of the burst, this "PWM" (Pulse WidthModulation) action results in an integration with finer resolution thandiscrete counting (mentioned above) can provide.

Method 2: Integration of Peaks

This method is completely different from the pulse integration mentionedabove. In this method, the ring signal is input to a sample and holdcircuit. The output from the sample and hold circuit is then integratedby an RC network. This approach basically integrates or represents theenergy under the curve for the ring signals. The advantage to this iseven finer resolution of secondary impedance as compared to pulseintegration. With reference to FIG. 12, it can be seen that there are nosteps in the integration waveform as seen before in FIG. 11. Here, achange in secondary impedance changes the slope of the integrationrather than the number of steps. Greater accuracy is achieved by theapproach, because both the positive and negative peaks (from the ringsignal) are used.

Method 3: Analog/Digital Signal into Microprocessor

This is obviously the most elegant solution, which requires amicroprocessor on the column side. However, since the airbag ECU has aprocessor already, this technique should be considered if there issufficient computing power leftover in the airbag processor. Thistechnique involves passing the ring signal directly to an analog todigital (A/D) converter, and treating the results entirely in a digitalmanner (as opposed to the combination of analog and digital circuits inthe previous methods). This method has the highest level of accuracy andsystem diagnostic capabilities.

In terms of comparison, each method has advantages and disadvantages.The Comparator Threshold method, while requiring the least amount ofcircuitry, provides the lowest resolution. Improved resolution isprovided by the Peak Integration method, however more complex circuitryis required. The Analog/Digital Signal into a Microprocessor methodprovides the highest resolution and can also detect transformer gapvariation. This method, however requires the most complex circuitry andrequires additional circuit integration into the airbag ECU.

Horn Detection

The same "ring" phenomenon discussed for squib condition is also used todetect the status of the horn switch. Normally, the system is monitoringthe squib resistance and can detect the change in ring waveform when thehorn switch is depressed. The frequency or rate of the ringing ischanged by the horn switch connecting to a capacitor across thesecondary. This change is easily detected at the primary and a sample ofthe ring signal was shown in FIG. 8. It should be noted that the squibcondition cannot be "seen" while the horn switch is depressed. Thesolution to this is to latch the last squib value when the horn switchis detected and return to normal BIT once the horn switch is released.Airbag deployment is not affected by the horn switch status.

Operational Circuit Example

The following description is for only one of a number of ways toactually schematically process the ringing signal observed at theprimary of the rotary transformer. As noted above, the mainconsideration of this disclosure is the basic concept of pulsing thetransformer primary winding 100 and observing and processing theresultant ringing waveform to derive information about secondaryconditions. Simple processing circuitry can derive sufficientinformation for many requirements, but the principal, and resultingsignature lends itself to high accuracy and resolution if appropriateprocessing is performed.

FIGS. 13A-C are a schematic diagram of one type of circuit which employsthe principles described previously, including squib deploy, squib test,horn detection, and low average current consumption. On the secondaryside 106 of the transformer are shown the squib resistor 50, the twoparallel/reversed diodes, the ringing capacitor C4, and the horn switch200 and associated capacitor C10, all of course which would be locatedwithin the steering wheel assembly. All remaining circuitry is locatedon the vehicle end of the steering column, with the rotary transformer10 as the coupling element.

Immediately connected to the lower side of the primary winding is thedrain terminal of a power FET transistor Q1, which in thisimplementation is the main power inverter driver for the squib firingfunction. With the high side of the transformer tied to a 12 to 16 vdc,high current power source, and the gate of the transistor driven by a 4Khz square wave, substantial energy would be transferred to thesecondary, which, as described previously, would easily forward bias therectifier diodes and apply firing power to the squib element 50. Notobvious in FIG. 13A, during airbag deploy the 12-16 vdc high currentsupply is switched directly to the high side of the transformer justwhen Q1 is driven in the high power inverter mode at 1 to 4 Khz. A logicsection is also required to steer the gate input of Q1 from normal testmode signals over to the 4 Khz square wave signal source.

During normal operation only the squib condition testing and horn switchdetection functions are enabled. These are as follows:

Connected in series with one of the transformer are two resistors, R1 at22 ohms, and R2 at 470 ohms. The 470 ohm resistor has a capacitor C1, of0.15 mfd across it. The high side of this combination is connected to aregulated power source. Now, if the gate of Q1 is turned on from an offcondition, the drain terminal will quickly toggle toward ground level.Initially, capacitor C1 is at a zero charge level, effectively initiallybypassing the 470 ohm resistance, generating a current step with aleading edge magnitude limited mostly by the 22 ohm resistance of R1partially by the inductance it is driving, and which flows through theprimary winding. The time constant of the 22 ohm resistor and the 0.15mfd capacitor is such that the capacitor charges rapidly and the currentquickly decays to a lower level, determined by the higher, 470 ohm R2resistor. In effect, a brief current spike of controlled shape isimposed onto the primary winding, generating a voltage spike on thesecondary winding. The initial pulse is now generated, which excites thetransformer and related circuitry into a ringing mode. As describedearlier, the capacitor C4 resonates with the transformer inductance. Theresonance dampens out due to resistance (mainly the squib) in the loop,producing a ringing burst decaying to zero. The decay shape or "dampingfactor" is a function of the load resistance. The impedance of theprimary driving source is high enough to minimize its effect on damping.The same characteristic ring is generated each time Q1 is turned on.

During the ringing burst, Q1 holds the lower end of the primary windingat ground level. Thus the ringing voltage generated can be convenientlyobserved at the top of the winding with respect to ground. This circuittakes advantage of this by feeding the signal at this point into avoltage comparator, IC1a (LM393), and toggling the comparator with theringing as it swings positive and negative with respect to ground. Theinverting input at pin 6 is connected to ground, and the transformersignal fed to the non-inverting input through a small filter, R3 and C2.Whenever Q1 is off, which is most of the time, the non-inverting inputof the comparator is solidly held high through R2 and R1 up to thepositive supply voltage, thus the comparator at pin 7 is high, or at alogical "1" state. When Q1 is briefly pulsed on (approximately 500microseconds) to induce ringing, the ringing cycles that swing negativewith respect to ground then pull the non-inverting input below theground level on the inverting input and the comparator output toggleslow for that interval. A small dc offset voltage exists on thetransformer winding as a result of the current though the "on"resistance of the FET. This provides a specific "window" at thecomparator input for the ringing to oscillate around. Toward thediminishing end of the ringing, the negative swings can no longer exceedthis and the comparator output is no longer toggled. Thus, the ringingaction produces a burst of pulses at the comparator output each time Q1is gated on. As discussed above, one simple technique for processingthese bursts for identifying squib condition is to merely count thenumber of comparator output pulses each time the transformer is rung.The circuit described here, however, instead integrates the burst toproduce a dc voltage signal. Here the output of the comparator is fed tothe base of a PNP transistor Q2 (FIG. 13B), which switches on duringeach negative pulse. The emitter of Q2 is tied to the regulated voltagesupply. The collector of Q2 now pulses regulated voltage into anintegrated network, R4-C3. The integrating capacitor, C3, accumulates acharge based on the number of and width of the pulses from Q2, such thatthe voltage across it at the end of the sampling period isrepresentative of the energy in the ringing burst. This method providesbetter resolution than pure pulse counting because it is furtherinfluenced by a certain amount of pulse width modulation of thecomparator output signal caused by the diminishing amplitude of thedecaying waveform. The integrated voltage produced on C3 from eachringing burst is retained as a DC voltage which is a function of theload resistance (squib) on the secondary. It is held in a sample andhold manner (for other processing) until just prior to the acquisitionof the next burst where it is dumped to near zero level in preparationfor the next integration cycle. Transistor Q3 is used to dump the chargeon C3. The base of Q3 is driven by the output of monostable timer IC3bin the following manner. This timer is initiated each time the ringingpulse is triggered by the main timing generator IC4a&b. The time periodis selected to be slightly shorter than the main timer such that C3 willalways be dumped just prior to a new integration period. For instance,if the main cycling operates on a 200 millisecond cycle (5 times persec), then this timer is set for, say, 180 milliseconds. The first brief500 microseconds (or 1/2 millisecond) of this time would be theacquisition or integration time, then C3 would remain charged for approx180 milliseconds, during which it could be fed to voltage comparators,A/D inputs, etc. for processing, then the approximate remaining 20milliseconds would be the time Q3 is turned on to dump C3. If the maintiming cycle were to be sped up, then this time would have to beshortened accordingly. This method is only one of numerous ways tosample and hold the capacitor voltage signal, therefore the concept iswhat is important here, not the specific technique. It should also bementioned that whatever technique is used for processing, the first ringpulse could be blanked out of the measurement since, as describedearlier, this is always the same since it is from the impulse from thedriving circuitry, and only the subsequent ringing is significantlyaffected by the reflected load.

When the horn switch on the steering wheel is closed, it connects anadditional capacitor into the secondary circuit that lowers the ringingfrequency. This change is easy to detect with a variety of methods. Theparticular circuitry shown here uses a monostable timer IC2a, configuredin a retriggerable mode. If a negative pulse is applied to pin 5, theoutput at pin 6 toggles high and remains so until it times out afterwhich pin 6 goes back low. Once triggered, but prior to time out, ifanother trigger pulse is fed to pin 5, the monostable timing period willrestart, with pin 6 output staying high. Consequently, if successivelyretriggered at a rate frequent enough prevent timeout, the output wouldremain high. There would be a precise rate above which the output wouldstay high and below which the output would exhibit toggling. Thisfrequency would be determined by the reciprocal of the monostable timeperiod as determined by resistor R7 and capacitor C4. This effect isused as a sort of frequency or rate counter for the ringing pulses fromthe output of the comparator. Normal ringing rates of approx 25 Khzwould keep the output of IC3a, pin 6, high during the 500 microsecondringing period, in effect producing a single stretched pulse. In theevent that the horn is depressed, the ringing frequency would be loweredbelow the critical rate and the output at pin 6 would produce severalpulses, one for each ring pulse.

Now, although the existence of a single pulse versus a string of pulsesat the output of IC2a provides an effective frequency discriminator,this only occurs for approx 500 microseconds out of every 200milliseconds and must be further processed to provide a discrete logicsignal annunciating the horn condition. Again, numerous methods exist toaccomplish this. One way, shown here, uses another resettable monostabletime IC2b, which is triggered by the output of the frequency detectingmonostable IC2a, described above. Remembering that when IC2a isdetecting normal ringing (no horn), its output at pin 6 generates only asingle pulse during each 500 microsecond ring interval, thus meaningthat IC2b is only triggered once, by the first positive slope of theIC2a output. Now, if the IC2b timer interval is set to a period slightlyshorter than the main 500 microsecond period, its output at pin 10,which was toggled high at the start of the 500 microsecond interval,will go back low slightly before the 500 microseconds expires . Thissignal is fed to the "D" input, pin 5 of a CMOS 4013 "D: type flip-flop,IC3a. The complement of the 500 microsecond pulse is fed to the clockinput, pin 3, or this flip-flop. The "Q" output, pin 1, of the "D"flip-flop, as configured, will latch at the same state existing on the"D" input when the clock input is toggled high. Therefore, on eachcycle, the "D" input will be at zero state when the clock input goeshigh, generating a zero state on the flip-flop output. The overallresult is that the flip-flop output at pin 1 will remain continuouslylow as long as the ringing occurs at its normal frequency.

If the horn is enabled, the lower ringing frequency will cause theoutput of the frequency discriminating stage IC2a, to toggle more thanonce. In doing so, each positive transition retriggers the input ofIC2b, restarting its monostable timing interval each time. Thus theoutput at pin 10 now remains high at the end of the main 500 microsecondperiod. With the "D" input of IC3a now at a high state when the clockinput at pin 3 toggles high, the output at pin 1 latches high. Theresult is that the output at pin 1 remains steadily high while the hornswitch is closed and can be used as a command signal for the hornsystem.

The main timing pulses are generated here with two monostable timers,IC4a and IC4b. IC4b provides the 500 microsecond test period duringwhich the ringing is initiated and subsequently processed. IC4aestablishes the off duration between test pulses, or essentially therate of testing, i.e., when at 200 milliseconds the rate would be 5tests per second. Resistor R14 and capacitor C7 determine the time forIC4a and R15 and C8 and IC4b timing. The two timers alternately triggereach other on and off to form a free running timing generatoralternating between 500 microseconds and 200 milliseconds, the outputsof each being used as signal sources for the various processing circuitspreviously discussed.

The power supply regulator IC5 is a type that draws low quiescentcurrent to help minimize battery loading. It should be noted that all ofthe test waveforms shown in all of the above figures were with a 12 vdcsource, but behavior with 9 vdc would be essentially the same. Theactual power consumed by the entire circuit is very low, particularlybecause low power, very brief test pulses are used, and need only occurat a slow rate of 5 to 10 times a second, mainly determined by howfrequently the horn switch state is to be sampled. The logic blocks areCMOS type IC's with very low quiescent current and slow clock speeds.This permits operation with the vehicle shut down, but where horn switchdetection is still required.

Another type of processing circuit is shown in FIG. 14. This could beused in place of the sample and hold integrator system describedearlier. It is based on linearly processing the ringing signal,precision rectifying it, and integrating or adding the individual peakamplitudes of each of the rings as a summed voltage across a sample andhold capacitor. Elaborating, the ringing signal across the floatingtransformer primary winding is amplified and converted to a single endedsignal (with respect to ground) by the Differential Amp. stage. Thesignal is then rectified by a Precision Full Wave Rectifier stagewhereby both the positive and negative ringing waveforms swing in thesame polarity direction. Now both positive and negative rings areprocessed, doubling resolution. The peaks of these are now steeredthrough an integrated circuit "analog switch" to a Sample and Hold stagethat pumps up its capacitor voltage in steps, each step being equal tothe peak magnitude of each successive ring throughout a ring burst. Withthe decaying burst, each voltage step will be slightly smaller, but willstill add to the total voltage. The final voltage thus is representativeof the sum of the peaks of the ring, thus providing an accuraterepresentation of the shape of the ring. This sum can then be fed toappropriate comparators or A/D signal inputs for subsequent processing.This circuit also provides a control logic stage that interfaces withthe main timing and pulse generator signals described for the earliersystem. Also, the logic provides timing to exclude the first "fixedamplitude" ring from the integration process, improving resolutionsomewhat.

Yet another method of processing would be with a microprocessor system,sampling the decaying ringing signal at several points, through an A/Dconverter. Appropriate software would permit deriving accuratemeasurement of the damping characteristic and thus the resistance of thesquib load on the secondary. This method could compensate for largechanges in transformer gap, and indeed could permit determining theactual gap size by comparing the amplitude of the driving pulse to thatof the first ring. A microprocessor system could also generate allnecessary timing, pulses, logic, etc. Also, detection of ringingfrequency changes from the horn function, and other switches, would beeasily accomplished.

The invention has been described with reference to the preferredembodiments. Obviously modifications and alterations will occur toothers upon a reading an understanding of the specification. It is ourintention to include all such modifications and alterations insofar asthey come within the scope of the appended claims or the equivalentsthereof.

Having thus described our invention, we now claim:
 1. A method formonitoring through a vehicle steering column an operability of an airbag firing circuit comprising an RC network including a firing squib asan element of the network comprising steps ofimpressing a test pulse onthe firing circuit; generating a ring signal in the network in responseto the test pulse wherein the ring signal is representative of a stateof the air bag firing circuit; and, monitoring the ring signal bycomparing the ring signal with predetermined standards for identifyingwhether the state is indicative that the air bag firing circuit isoperable.
 2. The method as defined in claim 1 wherein the air bag firingcircuit comprises a column side and a wheel side connected by anelectrical link and said impressing comprises generating the test pulseon the column side and communicating the test pulse to the wheel sidethrough the electrical link.
 3. The method as defined in claim 2 whereinthe generating comprises generating the ring signal on the wheel sideand communicating the ring signal back to the column side through theelectrical link.
 4. The method as defined in claim 2 wherein theelectrical link comprises a contactless device.
 5. The method as definedin claim 1 wherein said monitoring comprises comparator thresholdprocessing of the ring signal by counting peaks therein for apredetermined window.
 6. The method as defined in claim 5 wherein saidprocessing includes integrating the ring signal.
 7. The method asdefined in claim 1 wherein said monitoring comprises inputting the ringsignal to a sample and hold circuit and integrating an output from thesample and hold circuit.
 8. The method as defined in claim 1 wherein ahorn switch is disposed as a part of the air bag firing circuit anddepressing of the horn switch by a vehicle operator modifies the ringsignal in a manner so that the monitoring recognizes the depressing andgenerates a signal for activating a vehicle horn.